Biological variation in cerebral asymmetry is associated with behavioral variation in linguistic function. Identifying the relative environmental and genetic contributions to the normal distribution of cerebral asymmetry is a critical step in identifying specific causal factors that facilitate or limit verbal ability. Twins can be studied to determine whether the cerebral asymmetry and verbal ability correlation is attributable to genetic factors, environmental factors, or a combination of these influences. This study will examine the additive genetic, shared and unique environmental contributions to a structurally and functionally related set of brain asymmetries. These brain asymmetries have been associated with oral and written language performance and include: planum temporale asymmetry in Wernicke's area, pars triangularis asymmetry in Broca's area, and cerebellar anterior lobe asymmetry. Central sulcus asymmetry will be studied as a non-language control measure. Monozygotic twins can also be studied to identify variables that produce phenotypic discordance. Preliminary observations suggest that perinatal risk factors affect the degree of neuroanatomical and behavioral similarity in twins. This longitudinal study will also determine if cerebral asymmetry changes with age and whether phenotypic discordance is exaggerated or diminishes with age. Results from this study will guide research designed to elucidate the relation between cerebral asymmetry and verbal ability towards genetic and/or environmental sources, may identify perinatal influences on brain development and behavior, will provide information regarding feasibility of molecular studies of cerebral asymmetry, and provide clinicians, patients and families a greater understanding of the biological origins for cognitive ability.